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- </div>
- <div class="section">
- <div class="titlepage"><div><div><h3 class="title">
- <a name="boost_typetraits.reference.has_complement"></a><a class="link" href="has_complement.html" title="has_complement">has_complement</a>
- </h3></div></div></div>
- <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">Rhs</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Ret</span><span class="special">=</span><span class="identifier">dont_care</span><span class="special">></span>
- <span class="keyword">struct</span> <span class="identifier">has_complement</span> <span class="special">:</span> <span class="keyword">public</span> <em class="replaceable"><code><a class="link" href="integral_constant.html" title="integral_constant">true_type</a>-or-<a class="link" href="integral_constant.html" title="integral_constant">false_type</a></code></em> <span class="special">{};</span>
- </pre>
- <p>
- <span class="bold"><strong>Inherits:</strong></span> If (i) <code class="computeroutput"><span class="identifier">rhs</span></code>
- of type <code class="computeroutput"><span class="identifier">Rhs</span></code> can be used in
- expression <code class="computeroutput"><span class="special">~</span><span class="identifier">rhs</span></code>,
- and (ii) <code class="computeroutput"><span class="identifier">Ret</span><span class="special">=</span><span class="identifier">dont_care</span></code> or the result of expression
- <code class="computeroutput"><span class="special">~</span><span class="identifier">rhs</span></code>
- is convertible to <code class="computeroutput"><span class="identifier">Ret</span></code> then
- inherits from <a class="link" href="integral_constant.html" title="integral_constant">true_type</a>,
- otherwise inherits from <a class="link" href="integral_constant.html" title="integral_constant">false_type</a>.
- </p>
- <p>
- The default behaviour (<code class="computeroutput"><span class="identifier">Ret</span><span class="special">=</span><span class="identifier">dont_care</span></code>)
- is to not check for the return value of prefix <code class="computeroutput"><span class="keyword">operator</span><span class="special">~</span></code>. If <code class="computeroutput"><span class="identifier">Ret</span></code>
- is different from the default <code class="computeroutput"><span class="identifier">dont_care</span></code>
- type, the return value is checked to be convertible to <code class="computeroutput"><span class="identifier">Ret</span></code>.
- Convertible to <code class="computeroutput"><span class="identifier">Ret</span></code> means
- that the return value of the operator can be used as argument to a function
- expecting <code class="computeroutput"><span class="identifier">Ret</span></code>:
- </p>
- <pre class="programlisting"><span class="keyword">void</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">Ret</span><span class="special">);</span>
- <span class="identifier">Rhs</span> <span class="identifier">rhs</span><span class="special">;</span>
- <span class="identifier">f</span><span class="special">(~</span><span class="identifier">rhs</span><span class="special">);</span> <span class="comment">// is valid if has_complement<Rhs, Ret>::value==true</span>
- </pre>
- <p>
- If <code class="computeroutput"><span class="identifier">Ret</span><span class="special">=</span><span class="keyword">void</span></code>, the return type is checked to be exactly
- <code class="computeroutput"><span class="keyword">void</span></code>.
- </p>
- <p>
- <span class="bold"><strong>Header:</strong></span> <code class="computeroutput"><span class="preprocessor">#include</span>
- <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">/</span><span class="identifier">has_complement</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code>
- or <code class="computeroutput"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">/</span><span class="identifier">has_operator</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code>
- or <code class="computeroutput"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code>
- </p>
- <p>
- <span class="bold"><strong>Compiler Compatibility:</strong></span> Requires working
- SFINAE (i.e. BOOST_NO_SFINAE is not set). Only a minority of rather old compilers
- do not support this.
- </p>
- <p>
- <span class="bold"><strong>Examples:</strong></span>
- </p>
- <div class="blockquote"><blockquote class="blockquote"><p>
- <code class="computeroutput"><span class="identifier">has_complement</span><span class="special"><</span><span class="identifier">Rhs</span><span class="special">,</span> <span class="identifier">Ret</span><span class="special">>::</span><span class="identifier">value_type</span></code> is the type <code class="computeroutput"><span class="keyword">bool</span></code>.
- </p></blockquote></div>
- <div class="blockquote"><blockquote class="blockquote"><p>
- <code class="computeroutput"><span class="identifier">has_complement</span><span class="special"><</span><span class="identifier">Rhs</span><span class="special">,</span> <span class="identifier">Ret</span><span class="special">>::</span><span class="identifier">value</span></code> is a <code class="computeroutput"><span class="keyword">bool</span></code>
- integral constant expression.
- </p></blockquote></div>
- <div class="blockquote"><blockquote class="blockquote"><p>
- <code class="computeroutput"><span class="identifier">has_complement</span><span class="special"><</span><span class="keyword">int</span><span class="special">>::</span><span class="identifier">value</span></code> is a <code class="computeroutput"><span class="keyword">bool</span></code>
- integral constant expression that evaluates to <code class="computeroutput"><span class="keyword">true</span></code>.
- </p></blockquote></div>
- <div class="blockquote"><blockquote class="blockquote"><p>
- <code class="computeroutput"><span class="identifier">has_complement</span><span class="special"><</span><span class="keyword">long</span><span class="special">></span></code>
- inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>.
- </p></blockquote></div>
- <div class="blockquote"><blockquote class="blockquote"><p>
- <code class="computeroutput"><span class="identifier">has_complement</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">int</span><span class="special">></span></code> inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>.
- </p></blockquote></div>
- <div class="blockquote"><blockquote class="blockquote"><p>
- <code class="computeroutput"><span class="identifier">has_complement</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">long</span><span class="special">></span></code> inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>.
- </p></blockquote></div>
- <div class="blockquote"><blockquote class="blockquote"><p>
- <code class="computeroutput"><span class="identifier">has_complement</span><span class="special"><</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">></span></code> inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>.
- </p></blockquote></div>
- <div class="blockquote"><blockquote class="blockquote"><p>
- <code class="computeroutput"><span class="identifier">has_complement</span><span class="special"><</span><span class="keyword">int</span><span class="special">*></span></code>
- inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">false_type</a></code>.
- </p></blockquote></div>
- <div class="blockquote"><blockquote class="blockquote"><p>
- <code class="computeroutput"><span class="identifier">has_complement</span><span class="special"><</span><span class="keyword">double</span><span class="special">,</span> <span class="keyword">double</span><span class="special">></span></code>
- inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">false_type</a></code>.
- </p></blockquote></div>
- <div class="blockquote"><blockquote class="blockquote"><p>
- <code class="computeroutput"><span class="identifier">has_complement</span><span class="special"><</span><span class="keyword">double</span><span class="special">,</span> <span class="keyword">int</span><span class="special">></span></code>
- inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">false_type</a></code>.
- </p></blockquote></div>
- <div class="blockquote"><blockquote class="blockquote"><p>
- <code class="computeroutput"><span class="identifier">has_complement</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">></span></code> inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">false_type</a></code>.
- </p></blockquote></div>
- <p>
- <span class="bold"><strong>See also:</strong></span> <a class="link" href="../category/value_traits/operators.html" title="Operator Type Traits">Operator
- Type Traits</a>
- </p>
- <p>
- <span class="bold"><strong>Known issues:</strong></span>
- </p>
- <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
- <li class="listitem">
- This trait cannot detect whether prefix <code class="computeroutput"><span class="keyword">operator</span><span class="special">~</span></code> is public or not: if <code class="computeroutput"><span class="keyword">operator</span><span class="special">~</span></code> is defined as a private member of <code class="computeroutput"><span class="identifier">Rhs</span></code> then instantiating <code class="computeroutput"><span class="identifier">has_complement</span><span class="special"><</span><span class="identifier">Rhs</span><span class="special">></span></code>
- will produce a compiler error. For this reason <code class="computeroutput"><span class="identifier">has_complement</span></code>
- cannot be used to determine whether a type has a public <code class="computeroutput"><span class="keyword">operator</span><span class="special">~</span></code>
- or not.
- <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">A</span> <span class="special">{</span> <span class="keyword">private</span><span class="special">:</span> <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">~();</span> <span class="special">};</span>
- <span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_complement</span><span class="special"><</span><span class="identifier">A</span><span class="special">>::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// error: A::operator~() is private</span>
- </pre>
- </li>
- <li class="listitem">
- There is an issue if the operator exists only for type <code class="computeroutput"><span class="identifier">A</span></code> and <code class="computeroutput"><span class="identifier">B</span></code>
- is convertible to <code class="computeroutput"><span class="identifier">A</span></code>.
- In this case, the compiler will report an ambiguous overload.
- <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">A</span> <span class="special">{</span> <span class="special">};</span>
- <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">~(</span><span class="keyword">const</span> <span class="identifier">A</span><span class="special">&);</span>
- <span class="keyword">struct</span> <span class="identifier">B</span> <span class="special">{</span> <span class="keyword">operator</span> <span class="identifier">A</span><span class="special">();</span> <span class="special">};</span>
- <span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_complement</span><span class="special"><</span><span class="identifier">A</span><span class="special">>::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// this is fine</span>
- <span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_complement</span><span class="special"><</span><span class="identifier">B</span><span class="special">>::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// error: ambiguous overload</span>
- </pre>
- </li>
- <li class="listitem">
- There is an issue when applying this trait to template classes. If <code class="computeroutput"><span class="keyword">operator</span><span class="special">~</span></code>
- is defined but does not bind for a given template type, it is still detected
- by the trait which returns <code class="computeroutput"><span class="keyword">true</span></code>
- instead of <code class="computeroutput"><span class="keyword">false</span></code>. Example:
- <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">/</span><span class="identifier">has_complement</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
- <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span>
- <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span>
- <span class="keyword">struct</span> <span class="identifier">contains</span> <span class="special">{</span> <span class="identifier">T</span> <span class="identifier">data</span><span class="special">;</span> <span class="special">};</span>
- <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span>
- <span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">~(</span><span class="keyword">const</span> <span class="identifier">contains</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="special">&</span><span class="identifier">rhs</span><span class="special">)</span> <span class="special">{</span>
- <span class="keyword">return</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">rhs</span><span class="special">.</span><span class="identifier">data</span><span class="special">);</span>
- <span class="special">}</span>
- <span class="keyword">class</span> <span class="identifier">bad</span> <span class="special">{</span> <span class="special">};</span>
- <span class="keyword">class</span> <span class="identifier">good</span> <span class="special">{</span> <span class="special">};</span>
- <span class="keyword">bool</span> <span class="identifier">f</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">good</span><span class="special">&)</span> <span class="special">{</span> <span class="special">}</span>
- <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> <span class="special">{</span>
- <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special"><<</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">boolalpha</span><span class="special">;</span>
- <span class="comment">// works fine for contains<good></span>
- <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special"><<</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_complement</span><span class="special"><</span> <span class="identifier">contains</span><span class="special"><</span> <span class="identifier">good</span> <span class="special">></span> <span class="special">>::</span><span class="identifier">value</span><span class="special"><<</span><span class="char">'\n'</span><span class="special">;</span> <span class="comment">// true</span>
- <span class="identifier">contains</span><span class="special"><</span><span class="identifier">good</span><span class="special">></span> <span class="identifier">g</span><span class="special">;</span>
- <span class="special">~</span><span class="identifier">g</span><span class="special">;</span> <span class="comment">// ok</span>
- <span class="comment">// does not work for contains<bad></span>
- <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special"><<</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_complement</span><span class="special"><</span> <span class="identifier">contains</span><span class="special"><</span> <span class="identifier">bad</span> <span class="special">></span> <span class="special">>::</span><span class="identifier">value</span><span class="special"><<</span><span class="char">'\n'</span><span class="special">;</span> <span class="comment">// true, should be false</span>
- <span class="identifier">contains</span><span class="special"><</span><span class="identifier">bad</span><span class="special">></span> <span class="identifier">b</span><span class="special">;</span>
- <span class="special">~</span><span class="identifier">b</span><span class="special">;</span> <span class="comment">// compile time error</span>
- <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
- <span class="special">}</span>
- </pre>
- </li>
- <li class="listitem">
- <code class="computeroutput"><span class="keyword">volatile</span></code> qualifier is not
- properly handled and would lead to undefined behavior
- </li>
- </ul></div>
- <p>
- <span class="bold"><strong>Known issues:</strong></span>
- </p>
- <p>
- For modern compilers (those that support arbitrary SFINAE-expressions and
- decltype/declval) this trait offers near perfect detection. In this situation
- the macro <code class="computeroutput"><span class="identifier">BOOST_TT_HAS_ACCURATE_BINARY_OPERATOR_DETECTION</span></code>
- will be defined after including <code class="literal"><boost/type_traits/has_complement.hpp></code>.
- Please note however, that detection is based on function signature only,
- in the case that the operator is a function template then has_complement
- cannot perform introspection of the template function body to ensure that
- the type meets all of the conceptual requirements of the actual code.
- </p>
- <p>
- For older compilers (<code class="computeroutput"><span class="identifier">BOOST_TT_HAS_ACCURATE_BINARY_OPERATOR_DETECTION</span></code>
- not defined) then there are a number of issues:
- </p>
- <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
- <li class="listitem">
- This trait cannot detect whether unary <code class="computeroutput"><span class="keyword">operator</span></code>~
- is public or not: if <code class="computeroutput"><span class="keyword">operator</span></code>~
- is defined as a private class member of type T then instantiating <code class="literal">has_complement<T></code>
- will produce a compiler error. For this reason <code class="literal">has_complement</code>
- cannot be used to determine whether a type has a public <code class="computeroutput"><span class="keyword">operator</span></code>~ or not.
- </li>
- <li class="listitem">
- <code class="computeroutput"><span class="keyword">volatile</span></code> qualifier is not
- properly handled and would lead to undefined behavior
- </li>
- <li class="listitem">
- Capturless lambdas are not supported and will lead to a compiler error
- if has_complement is instantiated on the lambda type.
- </li>
- <li class="listitem">
- Scoped enum types are not supported and will lead to a compiler error
- if has_complement is instantiated on such a type.
- </li>
- </ul></div>
- </div>
- <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
- <td align="left"></td>
- <td align="right"><div class="copyright-footer">Copyright © 2000, 2011 Adobe Systems Inc, David Abrahams,
- Frederic Bron, Steve Cleary, Beman Dawes, Aleksey Gurtovoy, Howard Hinnant,
- Jesse Jones, Mat Marcus, Itay Maman, John Maddock, Alexander Nasonov, Thorsten
- Ottosen, Roman Perepelitsa, Robert Ramey, Jeremy Siek, Robert Stewart and Steven
- Watanabe<p>
- Distributed under the Boost Software License, Version 1.0. (See accompanying
- file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
- </p>
- </div></td>
- </tr></table>
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